In depositions of dielectric films, it is often desirable to form a highly conformal layer that has good insulating properties (i.e., a low k-value), and good film quality (e.g., a high film density, and low wet etch rate ratio (WERR)). Unfortunately, there are few (if any) starting materials that combine all these qualities in a simple deposition. In silicon oxide dielectric depositions, highly conformal films typically have good flow characteristics that allow the film to migrate into gaps, voids and seams. However, oxide films with good flow characteristics also tend to have high water and silanol (i.e., Si—OH) concentrations, which increase the k-value and WERR of the film. On the other hand, depositions of low-moisture oxide films typically have lower k-values and WERRs, but are also more prone to forming gaps and seams due to their reduced flowability.
One way to mitigate the deficiencies between high and low moisture silicon oxide films, is first to deposit a conformal high moisture film and then anneal it to remove at least a portion of the water. Two conventional annealing methods are: (1) high-temperature thermal annealing, and (2) high-density plasma annealing. In a thermal anneal, the deposited oxide layer is raised to a temperature where a significant amount of moisture is evaporated out of the layer. Silanol groups are also broken down into water and Si—O bonds, with at least some of this water also escaping from the oxide layer. The result is an annealed silicon oxide layer that is more dense and more electrically insulating (i.e., having a lower k-value) than the initially deposited oxide film.
Conventional thermal anneals are more efficient when the anneal temperature is higher. A high-temperature anneal at over 1000° C. breaks down silanol bonds and evaporates moisture from a deposited oxide layer at a significantly higher rate than a 300° C. anneal. The higher removal rate shortens the anneal time and increases the efficiency of the anneal step. However, higher temperature anneals have to be balanced against thermal budget constraints in the fabrication process. For example, if the thermal anneal is being performed on an inter-metal dielectric (IMD) layer deposited over metal lines, the temperature ceiling for the anneal may be 400° C. or less. In some instances, thermal budgets that low can make thermal annealing impractical due to the long anneal times.
When high-temperature anneals are impractical, a second annealing method involving a high-density plasma may be used. In this method, the initially deposited silicon oxide layer is exposed to a high-density plasma typically formed from the breakdown of inert gases like helium and argon. Charged particles from the plasma strike the oxide film and cause the disruption of silanol bonds and removal of water vapor. Anneal temperatures in high-density plasma are generally lower than for thermal anneals, and can be used to anneal oxide films with low thermal budget constraints.
The highly energetic plasma particles can also disrupt carbon-silicon and carbon-carbon bonds in the oxide film. When depositing a pure silicon oxide layer, the breakdown and removal of carbon is a desirable outcome for a plasma anneal. However, for low-k oxide films that incorporate carbon to lower the dielectric constant of the material, carbon removal by the plasma can damage the film by increasing its k-value. Thus, there is a need for additional annealing methods that can efficiently cure dielectric films at low temperatures without adversely affecting the dielectric constant of low-k materials. This and other issues are addressed by embodiments of the present invention.